The principal objectives of this project are to investigate the role of the serum lipoproteins in cellular cholesterol transport and metabolism. Plasma lipoproteins are isolated and specifically labeled with 125I and 14C or 3H cholesterol or other lipids. The interactions of these labeled lipoproteins with cells growing in tissue culture is being investigated. Particular attention is being paid to the role of the various lipoprotein fractions in the process of removal of cholesterol from peripheral tissues.